How do we actually sit on a utility chair?

It might sound daft but how so we sit on a chair? when we draw images of a chair they are usually drawn at right angles, that the angle joining the seat and the back is at a 90 degrees. Yet in reality this is far from the truth. The images below show a utility chair from the studio, and me sitting on it, showing how the chair fits the arch of the back, and the natural way to sit.